SEPTENTRIO PolaRx5TR, Справочное руководство

Страница: 62 / 512

2 Operation Details
The accuracy of the PVT depends on:
• The signal level.
• The geometry of the satellite constellation expressed in the DOP values: these values
indicate the ratio of positional errors to range errors and are computed on the basis
of the error propagation theory. When the DOP is high, the accuracy of positioning will
be low.
• The number of available satellites: the more satellites are available, the lower the DOP.
Measurement redundancy also enables better outlier detection.
• Multipath errors on the pseudorange measurements: multipath errors can
be largely attenuated by enabling the APME multipath mitigation method
(see
setMultipathMitigation
) and/or using code smoothing (see
setSmoothingInterval
).
• The PVT mode as set by the
setPVTMode
command.
• The data available to compute ionospheric delays (see
setIonosphereModel
).
• The choice of the dynamics model: if the dynamics parameter set by the
setReceiverDynamics
command does not correspond to the actual dynamics of
the receiver platform, the position estimation will be sub-optimal.
The a-posteriori accuracy estimate of the computed position is reported in the variance-
covariance matrix, which comes in the
PosCovCartesian
and
PosCovGeodetic
SBF
blocks. This accuracy estimate is based on the assumed measurement noise model and may
differ from actual errors due to many external factors, most of all multipath.
2.4.1 SBAS Positioning
SBAS, which stands for ’Space Based Augmentation System’, enables differential operation
over a large area with associated integrity information. System errors are computed from a
dataset recorded over a continental area and disseminated via a geostationary satellite. The
operation of SBAS is documented in the RTCA DO 229 standard. SBAS improves over DGPS
corrections, in that it provides system corrections (ionosphere corrections and ephemeris
long-term corrections) next to range corrections (the "fast corrections" in the DO 229 termi-
nology).
The receiver provides an SBAS-aided position when it has sufficient satellites with at least
fast and long-term corrections. The corrections are used as long as their applicability has not
timed out. During the time-out interval the receiver applies correction degradation using the
information received in message type (MT) 07 and 10.
By default, the receiver selects the SBAS satellite with the most SBAS corrections available.
With the
setSBASCorrections
command, it is possible to force the receiver to use a par-
ticular SBAS satellite or a particular SBAS provider.
2.4.2 DGPS Positioning
DGPS (Differential GPS) is a pseudorange-based positioning technique where GNSS system
errors are reduced by the use of range corrections. To work in DGPS rover mode, the receiver
needs to receive differential corrections in the RTCM or CMR format.
62